An electrostatic discharge (ESD) event refers to a temporary and abrupt flow of current between two objects of differing electrical potentials. ESD can be a serious issue for a solid-state electronic integrated circuit (IC) as large potential changes and current flows that occur during the ESD event can damage silicon junctions and oxide insulators. Damage to an IC from the ESD event can diminish the performance of the silicon-based IC, if not render the IC inoperable.
A charge buildup on an IC can occur for a variety of different reasons and can occur during manufacturing, assembly, testing, and/or use of the IC. As a result, the IC can be subjected to inadvertent ESD events prior, and subsequent, to assembly and sale. ESD device testing is used to ascertain the sensitivity of an IC to an ESD event. ESD testing can be performed using a variety of approaches, with each approach attempting to model a different type of ESD event.
One ESD testing approach is based upon a charged device model (CDM). The CDM attempts to duplicate an ESD event that begins with an IC accumulating charge. The charged IC subsequently makes contact with a conductive object and discharges the accumulated charge into the conductive object. For example, during manufacturing, ICs can be charged while sliding down the feeder of an automatic assembler and subsequently discharged upon contact with an insertion head of the assembler.
ESD protection schemes are implemented to protect devices internal to the IC that can be vulnerable to ESD events. Devices considered vulnerable typically are devices that output to, or come in contact with, external nodes of the IC. For this reason, ESD protection schemes typically attempt to steer current around output devices, effectively bypassing the ESD sensitive output devices of the IC. In some cases, however, active devices internal to an IC, i.e., non-output devices, that neighbor unused substrate regions also can fail when the IC is subjected to a CDM type of ESD event. The failure of an active device due to a CDM type of ESD event can render the IC inoperable, thereby reducing manufacturing yields and increasing manufacturing costs for the IC.